This invention relates to a method for the production of electrically conducting, IR reflecting, fluorine-doped tin oxide layers on the surface of glass or ceramic objects or of enamel coatings, and more particularly, this invention concerns a method of applying a solution containing a mixture of alkyltin oxide and trifluoroacetic acid to such a surface, wherein the surface has been heated to 400.degree. to 700.degree. C. The invention also relates to an appropriate composition for the application of electrically conductive, IR reflecting layers on the surface of glass, ceramic or enamel for the implementation of the method of the invention.
It is well known that fluorine-doped tin oxide layers on surfaces of glass, ceramic or enamel decrease the electrical resistance of the coated surfaces and increase the infrared reflection. To produce such tin oxide layers, combinations of suitable tin compounds with fluorine-containing substances or fluorine-containing tin compounds are brought into contact with surfaces heated to 400.degree. to 800.degree. C. A coherent tin oxide layer, which is doped with fluorine, is formed on the glass, ceramic or enamel surface. The fluorine doping is of decisive importance for the desired properties of the coated surfaces, such as increased conductivity and IR reflection.
Spraying solutions of suitable tin and fluorine containing compounds onto surfaces to be coated is a particularly simple method for the application of fluorine-doped tin oxide layers.
In German Offenlegungsschrift 22 46 193, a method for the production of transparent, electrically conducting films on glass surfaces is disclosed. For this method, a solution of an organotin salt of trifluoroacetic acid in methyl ethyl ketone is used. These organotin salts are prepared, for example, from tin(II) oxide, dibutyltin diacetate, dibutyltin oxide or tributyltin oxide, by heating for several hours with an excess of trifluoroacetic acid or other perfluorinated monocarboxylic acids. The excess perfluorinated acid and the water of reaction are thereupon distilled off and the residue is purified by recrystallization. The composition for applying fluorine-doped tin oxide layers in the claimed method is then prepared from the purified tin carboxylate compounds by dissolution in methyl ethyl ketone. Thus a method of several steps is required to prepare the coating solutions.
Tin carboxylates, prepared by the method described above, have a high fluorine content, which is invariable due to the molecular structure of these compounds. However, a high fluorine content in starting materials for the production of electrically conducting tin oxide layers on glass, ceramic or enamel surfaces brings about excessive doping with fluorine and causes a distinct deterioration of the properties desired in such laYers. For example, the surface resistance values attained by such tin oxide layers are between 32 and 65 ohm/square. However, lower surface resistance values are desired.
Because of the given invariable molecular structure, it is not possible to adjust the fluorine doping in the layers to an optimal value using the materials disclosed in German Offenlegungsschrift 22 46 193, namely the tin carboxylates of trifluoroacetic acid or the analogous higher perfluorinated monocarboxylic acids and the compositions produced from these materials for the surface treatment of glass.